Recently, a distance measuring apparatus has been used as a device for determining the distance to an object (i.e., the object of range finding) based on a parallax between multiple imaging optical systems. Specifically, such a device has been used to determine the distance between running cars and as a member of an autofocusing system for cameras or a three-dimensional shape measuring system.
Such a distance measuring apparatus includes two lenses that are arranged side by side either horizontally or vertically and an imager with two image capturing areas that are associated with the two lenses, respectively. The two lenses produce images on their associated image capturing areas and the distance to the object can be determined by carrying out triangulation based on the parallax between those two images.
FIG. 23 illustrates how a distance measuring apparatus carries out triangulation. In FIG. 23, illustrated are a first imaging optical system with an image capturing lens L1 and a second imaging optical system with an image capturing lens L2. These imaging optical systems are arranged so that respective optical axes a1 and a2 of the first and second imaging optical systems run parallel to each other with a predetermined interval B left between them. A line segment that connects together the intersection between the optical axis a2 of the second imaging optical system and an image capturing plane N2 and the intersection between the optical axis a1 of the first imaging optical system and an image capturing plane N1 is called a “base line”, which is a line segment that never varies according to the position of the object and is used as a reference for triangulation. The length of that base line is equal to the interval B. Thus, the base line length will be identified herein by “B”.
The image of the object O of range finding is produced by the image capturing lenses L1 and L2 on the image capturing planes N1 and N2, respectively. In FIG. 23, a point P on the object O of range finding is supposed to be a measuring point. If the point P is located on the optical axis a1 of the first imaging optical system, the image of the point P will be produced at the intersection between the image capturing plane N1 and the optical axis a1 of the first imaging optical system. On the image capturing plane N2, on the other hand, the image of the point P will be produced at a distance Δ from the intersection between the image capturing plane N2 and the optical axis a2 of the second imaging optical system. This distance is called a “parallax” and its magnitude is called the “magnitude Δ of parallax”.
Supposing the focal length of the image capturing lenses L1 and L2 of the first and second imaging optical systems is identified by f, the following approximation equation is satisfied:
                    Δ        ≈                  B          ·                      f            Z                                              (        1        )            
The images produced on the image capturing planes N1 and N2 are subjected to correction, division and other kinds of processing so as to be processed easily by computational processing. By making pattern matching between the images produced on the image capturing planes N1 and N2 after they have been subjected to those kinds of processing, the amount of parallax Δ can be obtained. And by substituting the amount of parallax Δ calculated, the base line length B and the focal length f into Equation (1), the distance Z can be obtained.
Patent Document No. 1 discloses a distance measuring apparatus that uses a positive meniscus simple lens, having a convex surface on its object plane, to increase the focal length without increasing the overall length of the lens.
On the other hand, Patent Document No. 2 discloses modified lens configurations that were researched to provide an image capturing lens with good telecentric property and an easily adjustable lens aberration.
Citation List
Patent Literature
                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2003-15029        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2002-98885        